Spritzing a sunscreen ingredient into the stratosphere could help counteract the effects of global warming, according to scientists behind an ambitious new geoengineering project.

The plan involves using high-altitude balloons to disperse millions of tons of titanium dioxide—a nontoxic chemical found in sunscreen as well as in paints, inks, and even food.

Once in the atmosphere, the particles would spread around the planet and reflect some of the sun's rays back into space.

About three million tons of titanium dioxide—spread into a layer around a millionth of a millimeter thick—would be enough to offset the warming effects caused by a doubling of today's atmospheric carbon dioxide levels, according to project leader and chemical engineer Peter Davidson.

The idea was inspired by the 1991 eruption of Mount Pinatubo, a volcano in the Philippines, said Davidson, head of the U.K. consulting firm Davidson Technology.

That eruption spewed 20 million tons of sulfur dioxide into the stratosphere, which formed a fine mist of sulfuric acid that reduced global temperatures by about a quarter of a degree Fahrenheit (half a degree Celsius) for two years.

But sulfuric acid degrades the ozone layer and may trigger droughts, because it absorbs as well as scatters light, lowering temperatures enough to possibly disrupt circulation in the stratosphere, Davidson said.

By contrast, titanium dioxide is seven times more effective at scattering light. That means much less would be needed to achieve the desired effects, he said, and so "there will be a much lower impact on atmospheric circulation."

Pump Up the Slurry

For Davidson's project, a slurry containing titanium dioxide would be pumped skyward via flexible pipes, which would be hoisted aboard unmanned balloons flying about 12 miles (20 kilometers) high. A "hypersonic nozzle" would then spray the slurry as fine particles into Earth's upper atmosphere.

The balloons would be launched from ships or islands located in equatorial regions where storms are infrequent, to reduce the risk of lightning strikes and strong winds damaging the balloons, Davidson said.

What's new about Davidson's plan is the use of titanium dioxide and the balloon-dispersal system, which could make the effort cheaper than using previously suggested aircraft or rockets, said Rob Jackson, an environmental scientist at Duke University in North Carolina.

In any particle-dispersion system, "the biggest expense is getting the chemical up into the stratosphere," said Jackson, who is not involved in the new project.

And if such a project is deployed, it will need to be kept running for as long as atmospheric concentrations of greenhouse gases, such as carbon dioxide, remain high.

"We have to keep doing this until we go carbon negative," Jackson said. Considering the rate at which greenhouse gas emissions are being reduced, "we could be in this business for centuries."

Project leader Davidson estimates that his balloon dispersal system would cost between U.S. $800 million and $950 million a year, plus $2 billion to $3 billion annually for the titanium dioxide.

Davidson adds that the environmental impacts of spraying titanium dioxide would be minimal.

And "many tests on exposure to titanium dioxide dusts have been done," he said in an email. "No evidence has been found for health hazards that I am aware of, and at these minute concentrations, issues are most unlikely."

Still, history has shown that "anytime we've injected chemicals into the atmosphere, we've been surprised by the chemistry that results," Duke's Jackson said.